According to Jagex:
"How can I increase my chances of getting good drops?
Several factors determine the value of the drops you can receive. These include, but are not limited to:
* Your Combat level.
* Your opponents' Combat level.
* The value of items you would lose on death.
* The value of items your opponent lost.
* How long you have survived in a dangerous area on the PvP world (continuously re-entering a safe area has a negative effect on your drop potential).
* How many players you have killed on the PvP world recently (if you kill several players in a short amount of time, the quality of your drops will go down).
* Whether you are in a hot zone.
Drops will improve if you are carrying at least 75,000 coins' worth of items (or 25,000 coins' worth on a free world) that you will lose if you die - your three most valuable items are not included in this. To get better drops, your victim must also lose at least 25,000 coins' worth of equipment.
Note that if you risk significantly more than 75,000 coins in items, your drops will not improve further, nor will they improve if your opponent loses 25 million coins' worth of items - the values are minimum boundaries.
In general, the most important factors are the value of items you are risking, the amount your opponent drops and how long you have survived (ideally in a hot zone). A good tactic is to let your drop value build up by training in a hot zone for a while (which will be especially beneficial if you have some brawling gloves), then try to get a kill or two for a better chance of some good drops. Then you can go back to training to let your drop value increase again.
Note that if you die, your drop value will temporarily increase to compensate you for your loss, but it won't get you the value of your lost items - it's not a sensible way to get good drops!"